I've checked ferry costs, and to be honest there is little difference in prices, assuming you avoid the obvious dates to/from Le Mans - and remember in June you have the main Le Mans race... 16th/17th June.
Costs are approx:
Portsmouth to Santander £210
Portsmouth to Bilbao £185
(Both plus cabin)
Caen to Portsmouth £150
The biggest difference seems to be the day of the week you travel. Anything mid-week is often cheaper, especially on the shorter northern France crossing - the longer Spain crossing seems to be fairly consistent in price..
The outbound ferry, to arrive Santander or Bilbao early afternoon, leaves Portsmouth at 08:45 am.
I'd go for an overnight hotel/B7B local to Portsmouth, as we did for The Long Way Up!
The Santander morning ferry departs on Saturday, the Bilbao ferry on Tuesday.
At the moment, Uncle Albert and I have a preference for Tues. 26th June out, return Tues. 10th July (the dates already suggested).
And we may swerve Classic Le Mans - opting for a last night at Le Château, as this year...
This still provides for anyone to join the run later, or leave earlier.
A good short trip could even be to return via Santander or Bilbao, and swerve France all together...